We were able to reproduce the issue and therefore we have logged it in our Public Issue Tracking System. Here you can subscribe and track the progress of fixing it.
The good news is that there is a workaround - setting the SlectedItem through the RadListBox SelectionHelper as shown in the attached project.
It would be great if you can give it a try and let us know if the workaround works for you.
Please, excuse us for the inconvenience. We have added points to your account for the report. Don't hesitate to contact us if you have other questions.